The field of sonic logging of boreholes in the oil and gas industry involves making acoustic measurements in the borehole at frequencies typically in the range 500 Hz-20 kHz. Below this range is typically considered as the seismic domain, above it the ultrasonic domain. A summary of the general techniques involved in borehole acoustic logging can be found in GEOPHYSICAL PROSPECTING USING SONICS AND ULTRASONICS, Wiley Encyclopedia of Electrical and Electronic Engineering 1999, pp 340-365.
One example of a sonic logging tool used by Schlumberger is the Dipole Sonic Imaging tool (DSI), shown in schematic form in FIG. 1. The DSI tool comprises a transmitter section 10 having a pair of (upper and lower) dipole sources 12 arranged orthogonally in the radial plane and a monopole source 14. A sonic isolation joint 16 connects the transmitter section 10 to a receiver section 15 which contains an array of eight spaced receiver stations, each containing two hydrophone pairs, one oriented in line with one of the dipole sources, the other with the orthogonal source. An electronics cartridge 20 is connected at the top of the receiver section 15 and allows communication between the tool and a control unit 22 located at the surface via an electric cable 24. With such a tool it is possible to make both monopole and dipole measurements. The DSI tool has several data acquisition operating modes, any of which may be combined to acquire waveforms. The modes are: upper and lower dipole modes (UDP, LDP)—waveforms recorded from receiver pairs aligned with the respective dipole source used to generate the signal; crossed dipole mode—waveforms recorded from each receiver pair for firings of the in-line and crossed dipole source; Stoneley mode—monopole waveforms from low frequency firing of the monopole source; P and S mode (P&S)—monopole waveforms from high frequency firing of the monpole transmitter; and first motion mode—monopole threshold crossing data from high frequency firing of the monopole source. One frequently observed problem in dipole logging is the propagation of a flexural signal from the source to the receivers along the tool itself. This signal, often known as a “tool arrival”, interferes with the detection of the corresponding signal that has propagated in the formation and so is highly undesirable. Approaches that have been taken to remove or reduce the occurrence of tool arrivals include the provision of a device or structure between the source and receivers which prevents propagation of the tool arrival (an “isolator”), and adoption of a receiver structure which delays or attenuates the tool arrival.
One form of isolator is found in tools in which the sources and receiver are found in two separate bodies connected by a relatively flexible connector such as a cable or flexible tube. An example of this is found in U.S. Pat. No. 5,343,001. Such an approach is effective in preventing the tool arrival from passing directly along the tool body from the source to the receiver but has the problem in that the tool cannot be used in any borehole which is not vertical, or nearly so. Since boreholes that are deviated from vertical are very common, such a tool has limited application. This structure also does not address the problem of a flexural signal coupling into the receiver structure from the borehole and then propagating along the receiver.
For tools in which the sources and receiver are connected in a relatively rigid structure (i.e. one which can operate in deviated boreholes), the approach has been to interpose an isolator between the source and receiver which interrupts the tool arrival path with a structure which delays and/or attenuates the flexural signal propagating along the tool body. In the DSI tool described above, the sonic isolation joint includes stacks of rubber and steel washers located around connecting members. This structure is the only connection between the transmitter and receiver, there being no continuous housing or tool body between the two. The sonic isolation joint is disclosed in more detail in U.S. Pat. No. 4,872,526.
Another form of isolator is a segmented structure in which the isolator is made up from a series of segments, each of which is connect only to its neighbors, there being some resilient or absorbent material at each joint. Examples of such structures are found in U.S. Pat. No. 5,229,553 which has a series of shells and spools, or in U.S. Pat. No. 5,728,978 which has a number of tubular members joined by interlocking lobes (see also SPE 56790 A Dipole Array Sonic Tool for Vertical and Deviated Wells, Lucio N. Tello, Thomas J. Blankinship, Edwin K. Roberts, Computalog Research. Rick D. Kuzmiski, Computalog Ltd., 1999 SPE Annual Technical Conference and Exhibition, Houston, Tex. 3-6 Oct. 1999).
As well as providing an isolator between the source and receiver, modifications to the structure of the receiver section itself have been proposed. In the DSI tool, for which the receiver housing provides the main structural strength for the tool, a combination of slots and apertures and mass loading rings are used to modify the acoustic behavior of the housing to reduce or delay to flexural (and other) tool arrivals. Further examples of this approach can be found in U.S. Pat. No. 4,850,450 and U.S. Pat. No. 5,036,945. In U.S. Pat. No. 5,731,550 the segmented structure applied to the isolator in U.S. Pat. No. 5,229,553 is also applied to the receiver section. However, since this is not a rigid structure, it may be necessary to also provide a housing or sleeve to make the tool able to operate in deviated boreholes. Other approaches to addressing this problem are discussed in PCT Application No. PCT/EB98/00646, published as WO99/56155, and incorporated herein by reference.
To date, no approach has been completely successful in removing or preventing flexural tool arrivals. It is an object of the present invention to provide a tool structure in which the problem of flexural tool arrival can be handled in a way which does not compromise the ability of the tool to make dipole measurements of the formation.